In recent years, an increase in the transmission capacity of a transmission line has been demanded in line with an increase in Internet traffic. To meet this demand, a wavelength division multiplexing technology (WDM) which is capable of transmitting data with different wavelengths bundled by a single core fiber has been introduced centering on the core network. Herein, where the WDM technology is employed, an optical coupling and splitting filter having a satisfactory wavelength selection property is needed since different wavelengths transmit respectively individual types of information.
In addition, since crosstalk between signals of different wavelengths becomes a factor of signal deterioration, it is necessary that the wavelength of a laser diode (LD) which is used as a signal source is stabilized in a passband of an optical coupling and splitting filter. In particular, in a Dense WDM (DWDM) technology having high density, since the pass band of the optical coupling and splitting filter is narrow, it is necessary to carry out wavelength monitoring and control.
Since the accuracy of monitoring and controlling the wavelength depends on an interval between wavelengths, the accuracy in wavelengths is made severe in line with narrowing of the interval between wavelengths. For example, in the DWDM technology used for the core network, the interval between wavelengths is mainly 200 GHz through 50 GHz (1.6 nm through 0.4 nm). In the near future, the interval will be made narrower still.
An oscillating wavelength of the LD is greatly influenced by temperature. Usually, a wavelength monitoring and controlling mechanism is provided in the interior of an optical transmitter module or an optical transmitter and receiver module. The wavelength monitoring and controlling mechanism feeds monitor output signals for monitoring and controlling wavelengths back to a temperature controller and carries out control so that the oscillating wavelengths are kept constant.
FIG. 12 is a general view of a prior art wavelength monitoring and controlling mechanism disclosed in, for example, “25 GHz-spacing wavelength monitor integrated DFB laser module” of Institute of Electronics, Information and Communication Engineers C-4-44, 2002, which was prepared by Takagi et.al., and the schematic shows one example of an optical system for monitoring and controlling wavelengths in which an etalon (or Fabry-Petrot optical resonator) is employed. In the drawing, reference number 12 denotes an optical fiber, 13 denotes a forward lens, 14 denotes a DFB-LD (a distributed feedback laser diode), 15 denotes a rearward lens, 16 denotes a prism, 17 denotes a temperature controller, 18 denotes an etalon, 19a and 19b denote optical detectors. A wavelength monitoring and controlling method using an etalon in an optical system for monitoring and controlling wavelengths is also disclosed in Japanese Patent Application Laid-open Nos. 2001-196689 and 2003-283044, and U.S. Pat. No. 6,353,623.
The DFB-LD 14 is installed centrally, and an optical transmission system for optical signals is shown at the arrow A side. A laser beam emitted from the forward end face is collimated by the forward lens 13 and is coupled into an optical fiber 12. On the other hand, an optical system for monitoring and controlling wavelengths of the DFB-LD 14 is shown at the side opposite to the arrow A. An LD beam emitted from the rearward end face is used for monitoring and controlling. The LD beam is collimated by the rearward lens 15 and is branched into two by the prism 16. One of the split LD beams is coupled directly to the optical detector 19a and the other thereof is made incident into the etalon 18. Output signal of the light made incident directly into the optical detector 19a are used for automatic optical output control.
Output signals made incident into two optical detectors 19a and 19b are used for monitoring and controlling wavelengths. Light passed through the etalon 18 is collimated and is made incident into the optical detector 19b. The resonator length of the etalon 18 is accurately adjusted so as to correspond to a wavelength to be monitored. Therefore, since the amount of outputted light changes in line with a fluctuation in the wavelength, a difference between the light amount and the output signal made incident into optical detector 19a is detected as a fluctuation in output of the optical detector 19b. The output is fed back to the temperature controller 17 of the LD light, thereby controlling the wavelength of the LD light. Thus, the wavelength is directly extracted in terms of hardware and is used for control.
On the other hand, a method for monitoring and controlling wavelengths, in which no etalon is employed for an optical system for monitoring and controlling wavelengths, has been developed. For example, in Japanese Patent Application Laid-open No. 1-235390(1989), a method for monitoring and controlling a wavelength, in which the relationship between an environmental temperature and a change in a wavelength (that is, an amount of wavelength deviation) is stored in advance and the temperature is controlled based on the relationship, is disclosed. In Japanese Patent Application Laid-open No. 2000-323785, a method for monitoring and controlling a wavelength, which stores in advance the data obtained by actually having measured the laser temperature with respect to the laser drive current and controls a laser drive current by predicting an actual amount of rise in temperature on the basis of the data, is disclosed as another example.
As described above, in order to suppress crosstalk which becomes a factor of signal deterioration, wavelength monitoring and controlling are indispensably necessary to stabilize the oscillating wavelength of a light source within the pass band of an optical coupling and splitting filter. However, where an optical filter such as an etalon is employed for a wavelength monitoring and controlling system, the optical system becomes expensive, and the number of assembling steps is increased, making it difficult to reduce the production costs. Also, since the etalon has temperature dependency (for example, Y. C. Chung et. al, “Synchronized etalon filters for standardizing WDM transmitter laser wavelength,” IEEE Photon. Technol. Lett., Vol., pp. 186-189, February 1993), for example, a Peltier device is requisite. As a result, it is difficult to make the wavelength monitoring and controlling system small in size. In addition, there is still another problem in that, since the temperature adjusting feature operates at all times to secure a reference temperature, power consumption for adjusting the temperature becomes high.
On the other hand, in the method for monitoring and controlling a wavelength, which does not employ an etalon filter in a prior art optical system for monitoring and controlling a wavelength, for example, in the cases of Japanese Patent Application Laid-open Nos. 1-235390(1989) and 2000-323785, the temperature is controlled using the relationship between the environmental temperature stored in advance and a change in the wavelengths (that is, an amount of wavelength deviation) without directly calculating the wavelength. Therefore, where a change in the wavelength (that is, an amount of the wavelength deviation) depends on factors other than the temperature, sufficient monitoring and control cannot be secured.